Revert "Fix Android back button behavior"

This reverts commit 7f29724a8561dee3e10306d280e8eb8c6c668bcc.

Conflicts:
	MonoGame.Framework/Android/AndroidGameActivity.cs

Conflicts:
	MonoGame.Framework/Android/AndroidGameActivity.cs
Fix AndroidGamePlatform.Exit() behavior
Merge pull request #2813 from flyingdevelopmentstudio/FixAndroidGamePlatformExit

Fix android game platform exit
Merge branch 'develop' of https://github.com/mono/MonoGame into ouya-1.0.13

Conflicts:
	MonoGame.Framework/Android/AndroidGameActivity.cs